Dysplasia epiphysealis hemimelica (DEH) is a rare developmental disorder of childhood and is characterized by asymmetric enlargement of the epiphyseal cartilage of the long bones. After 4 to 5 years of age, the lesions histologically resemble osteochondroma. To our knowledge, only one publication of this entity is available in an English pathology journal. The clinical, radiographic, and histologic features of 9 cases of DEH were retrospectively reviewed. The patients' age ranged from 3 to 15 years with single or multiple lesions of the femur, fibula, tibia, and talus. The etiology and pathogenesis of DEH are not known. Its origin and evolution has initially apparent similarities to the development and growth of epiphyseal secondary ossification centers. DEH can be differentiated from osteochondroma of long bones using clinical, radiologic, and pathologic parameters. DEH occurs in young children and adolescents manifesting as lesions that arise particularly from the epiphysis of the lower extremities and tarsus. Osteochondroma, in contrast, occurs most frequently between 10 and 30 years of age and originates from the metaphysis of long bones. Although the DEH cartilage resembles osteochondroma, there are several significant histologic differences. During infancy, lesions of DEH histologically reveal osteocartilaginous nodules that resemble secondary ossification centers. Usually after 4 to 5 years of age they develop into osteochondroma-like lesions. Although all cases of DEH contain small areas of calcified cartilage beneath the cartilage cap, a significant percentage of osteochondromas show large amounts. The nodules and cartilage cap of DEH contain bands of cartilage separating areas of cancellous bone; these bands are not present in osteochondroma. Among the other distinguishable features, recent molecular studies of DEH demonstrated normal expression levels of EXT1 and EXT2 genes, comparable to that of normal growth plate. Osteochondroma, in contrast, has low levels of EXT1 and EXT2 gene expression due to gene mutation. The histologic differences in combination with the distinct clinical and radiographic features should enable a pathologist to differentiate these entities.